The present invention relates to a vehicle steering apparatus, and more particularly, to a steer-by-wire steering apparatus. The present invention also pertains to a method for controlling the apparatus.
FIG. 7 is a diagrammatic view showing a conventional steer-by-wire vehicle steering apparatus. A manipulation mechanism 500 has a steering wheel angle sensor, not shown, for detecting the steering angle of a steering wheel 510 operated by the driver, and has a gear ratio converting section 540 for generating a steered wheel position command based on the steering angle, and passing it to a steering mechanism 600. The steering mechanism 600 drives a steering motor 610 in accordance with a steered wheel position command given from the manipulation mechanism 500, to steer the steered wheels T via a steering rod 620.
A steering axial force estimating section 700 estimates a steering axial force applied from the road surface to the steering rod 620 in a disturbance observer section 710, and generates a counter torque command to be applied to the steering wheel 510 in a counter torque command generating section 720. A counter torque motor control section 530 of the manipulation mechanism 500 drives a counter torque motor 520 in accordance with the counter torque command given from the steering axial force estimating section 700, and conveys a counter torque to the driver.
FIG. 8 is a control block diagram of a steering motor control section 630 in the steering mechanism 600. In this control block diagram, a steering motor position command value θc, is input in accordance with a steered position command given from the manipulation mechanism 500. A steering motor position control section 631 generates a current command value Ic to the steering motor 610, based on a deviation Δθ between the steering motor position command value θc and an actual steering motor position θr. A current control section 632 controls a PWM drive section 633 to have a motor applied voltage VPWM such that the actual current value Ir is matched with the current command value Ic, namely, a current deviation ΔI between current command value Ic and actual current value Ir is zero.
A disturbance observer section 710 of a steering axial force estimating section 700 calculates (estimates) a steering axial force Fdis applied to the steering rod 620 based on an angular velocity ωr obtained by differentiating the actual position θr of the steering motor 610, and a current command value Ic of the steering motor 610, as disclosed in Japanese Laid-Open Patent Publication No. 2002-274405. A constitution example of the disturbance observer section 710 is represented in the following expressions (1) and (2).
                              F          dis                =                              [                          Conversion              ⁢                                                          ⁢              factor              ⁢                                                          ⁢              from              ⁢                                                          ⁢                              T                dis                            ⁢                                                          ⁢              to              ⁢                                                          ⁢                              F                dis                                      ]                    ×                      T            dis                                              (        1        )                                          T          dis                =                                                            -                                  gs                                      s                    +                    g                                                              ·              J                        ⁢                                                  ⁢                          ω              r                                +                                    g                              s                +                g                                      ·                          K              t                        ·                          I              c                                                          (        2        )            
Where Fdis is the steering axial force, Tdis is the disturbance torque of the steering motor 610, s is Laplacian operator, g is the observer pole, J is the inertia of the steering motor 610, Kt is the torque constant of the steering motor 610, ωr is the angular velocity of the steering motor 610, and Ic is a current command value to the steering motor 610.
Conventionally, considering that the current command value Ic and the actual current value Ir are equal under the premises that the current control section 632 (see FIG. 8) of the steering mechanism 600 operates almost ideally, the current command value Ic is employed to calculate (estimate) the steering axial force Fdis. A counter torque command is generated in accordance with this steering axial force Fdis. However, when the current control section 632 does not have a sufficiently high gain, the current command value Ic and the actual current value Ir are not necessarily equal. In this case, if the steering axial force Fdis is estimated using the current command value Ic, an error might occur.
On the other hand, if the steering motor position control section 631 has a higher gain to enhance the position control performance of the steering motor, the current command value Ic may oscillate due to influence of a backlash or friction of a ball screw provided between the steering motor 610 and the steering rod 620, when the steering axial force Fdis applied on the steering motor 610 is reversed. If the steering axial force Fdis is estimated using this current command value Ic, an estimated steering axial force Fdis may contain some oscillation of the current command value Ic, resulting in a problem that the counter torque may oscillate. FIG. 9 shows an instance in which a counter torque command generated using the current command value Ic oscillates. In the figure, the A and B parts are oscillating.
When the driver continued to perform the steering in a state where the steering rod 620 is subjected to a large steering axial force Fdis from the road surface, the following problem arise. For example, when the steering wheel T is subjected to a large resistance due to a bump against the curbstone during the steering, a significant deviation occurs momentarily between the actual position of the steering motor 610 and the steered wheel position command. The steering motor position control section 631 generates a larger current command value Ic to eliminate this deviation. As a result, the current deviation ΔI momentarily increases, and the actual current value Ir of the steering motor 610 is not matched with the current command value Ic. At this time, the current control section 632 tries to flow the actual current according to the current command value Ic by increasing the applied voltage VPWM to the steering motor 610 via the PWM drive section 633.
When a large steering axial force Fdis is applied from the road surface, a larger current is required. However, since the maximum value of voltage applicable to the steering motor 610 is restricted by a battery voltage VB, if the duty ratio of PWM control becomes 100% and the battery voltage VB is applied (voltage saturation state), any more current cannot be flowed. As a result, the current command value Ic and the actual current value Ir of the steering motor 610 are not matched. When the actual current value Ir of the steering motor 610 does not match the current command value Ic, the position control performance of the steering motor 610 is lower, and the actual position of the steering motor 610 cannot follow the steered wheel position command. In this case, the steering axial force Fdis itself is correctly estimated by employing the actual current value Ir, instead of the current command value Ic, in the expression (1).
However, since the information that the motor applied voltage VPWM reaches the battery voltage VB, and that the steering motor 610 cannot follow correctly further turning of the steering wheel 510 is not conveyed to the driver at all, there is a problem that the steering motor 610 cannot follow if the driver continues to further turn the steering wheel 510 without being informed of anything.